1. Field of the Invention
The present invention relates to an apparatus and a method for interpolating lines of an input image signal, and more particularly, to an apparatus and a method capable of solving problems of a stepwise image or a saw-toothed image at the edge due to a time gap of two fields, when an interlaced scanning image signal formed with two fields is converted to a progressive scanning image signal. The present application is based on Korean Patent Application No. 2001-59106, filed Sep. 24, 2001, which is incorporated herein by reference.
2. Description of the Related Art
When a progressive scanning image signal is generated by extracting one frame image from a motion picture signal of an interlaced scanning video camera, a camcorder, a television, or a VCR (Video Cassette Recorder) having one frame formed with two fields, if the motion picture has a movement due to a time gap between a first field and a second field, or there is a movement in the image due to a hand tremor, then the difference between the fields is significant, and a stepwise image or a saw-toothed image occurs at an edge where the movement is generated.
To reduce the above phenomena, there is a need to interpolate an interlaced scanning image data appropriately via a process known as de-interlacing. There are two ways to interpolate: 1) generating data of one frame by interpolating lines of one field data; and 2) generating data of one frame by using data from more than two fields. Here, the first method of interpolating the lines of the data of one field and generating the data of one frame will be described. Moreover, disclosures in Korean Patent No. 0224859 to the applicant of the present invention will be introduced and a feature which is an improvement on the above Patent will be described.
FIG. 1 is a flow chart showing a conventional method for line interpolating. Referring to FIG. 1, the line interpolating method is comprised of the steps of: an edge component extracting step (S 100); a slope calculating step (S 110); an interpolating step (S 120); and a composition step (S 130). Here, a pixel value of the image signal data (hereinbelow, referred to as ‘input data’) is referred to as Xref[i,j], and the data generated after being interpolated (hereinbelow, referred to as ‘interpolation data’) is referred to as Xgen[i,j].
In the edge component extracting step (S 100), the edge component of a horizontal direction is extracted from the pixel value of the input data, Xref[i,j]. The edge component of the horizontal direction can be obtained by using a horizontal edge extracting filter. The simplest type of filter is a high-pass filter having a horizontal frequency as a center frequency. An edge component value corresponding to the pixel value of the input data is referred to as Eref[i,j]. The slope calculating step (S 110) is a step of obtaining a slope of the pixel included in the interpolation data from the edge component value, Eref[i,j], of the input data. This step can be realized with software using an information processing component or with hardware such as a slope detection circuit.
The pixel included in the interpolation data and the input data edge component values around the pixel have a spatial structure shown in FIG. 2. Here, the number of values of edge component data of the input data used in determining slope direction is decided at the discretion of the designer.
In the slope calculating step (S 110), pixels meeting the conditions described below are selected one by one for each of the upper and lower horizontal lines among the pixels included in the input data. First of all, a reference region of the pixel area which is to be interpolated is identified. In this region the pixel of the upper horizontal line having the maximum edge component and the pixel of the lower horizontal line having the maximum edge component are selected. The maximum edge component value of the selected pixel of the upper horizontal scanning line is referred to as Eup—max[i+du, j]. In addition, the maximum edge component value of the selected pixel of the lower horizontal scanning line is referred to as Edown—max[i+dd, j+1]. Here, ‘du’ is a horizontal delay between the pixel to be generated of the upper slope and the pixel having the maximum edge component in the upper horizontal scanning line of the reference region. On the other hand, ‘dd’ is a horizontal delay between the pixel to be generated of the lower slope and the pixel having the maximum edge component in the lower horizontal scanning line of the reference region. The value of ‘du’ increases to the right of FIG. 2, and is positive (‘+’) to the right of the pixel to be generated and is negative (‘−’) to the left of the pixel to be generated; the value of ‘dd’ increases to left of FIG. 2, and is positive (‘+’) to the left of the pixel to be generated and is negative (‘−’) to the right of the pixel to be generated.
Following is a discussion describing the establishment of two different statuses or states based on criteria related to Eup—max[i+du, j] and Edown—max[i+dd, j+1].
Status 1:
In status 1, all of the five conditions described below are met at the same time.|Eup—max[i+du, j]−Edown—max[i+dd, j+1]|<Eth  (Condition 1)
This condition means that an absolute value of the difference value of the maximum values of the edge component at the upper and lower horizontal lines is less than the predetermined value ‘Eth’.∥du|−|dd∥≦1  (Condition 2)
The above condition means that a relative position between the pixels to be referred is limited. More specifically, the condition means that the distance between the pixels to be referred should be within the distance of one pixel in a vertical or a diagonal direction.du×dd<0  (Condition 3)
This condition means that the pixels to be referred are placed in the diagonal direction, with a pixel to be generated being at the center.Eup—max[i+du, j]≧Eref[i,j]+a  (Condition 4)
The condition means that the difference of the edge component between the pixel to be referred and included in the upper horizontal line and the pixel to be referred and placed right above the pixel to be generated should be more than the predetermined value ‘a’.Edown—max[i+dd, j+1]≧Eref[i,j]+a  (Condition 5)
The above condition means that the difference of the edge component between the pixel to be referred and included in the lower horizontal line and the pixel to the referred and placed right above the pixel to be generated should be more than the predetermined value ‘a’. Here, ‘Eth’, and ‘a’ are constant values, which are appropriately decided by the designer in consideration of the value of the image signal and the frequency band of the image signal.
Status 2:
Status 2 covers all situations other than status 1.
Here, the statuses 1 and 2 are referred to as statuses to be interpolated.
FIG. 3 is a view showing the relation between the statuses to be interpolated (statuses 1 and 2), the pixel to be generated and the pixel to be referred. In the interpolating step (S 120), the value of the pixel to be generated is obtained by a vertical linear interpolation by using the result of the slope calculating step (S 110). The interpolating step (S 120) is performed as follows in accordance with the status to be interpolated.
1) Status 1 (state 1):
The value of the pixel of the image data to be interpolated, Xgen[i,j], is (Xref[i+du, j]+Xref[i+dd, j+1])/2.
2) Status 2 (state 2):
The value of the pixel of the image data to be interpolated, Xgen,[i,j] is (Xref[i,j]+Xref[i,j+1])/2.
In the composition step (S 130), a frame image is generated by compounding the result of the image generated by the interpolating step (S 120) with the image of the input data.
However, according to the conventional art described so far, when the image has an area of high frequency such as a minute pectinate or lattice pattern, the edge component detected in the horizontal edge detecting step and the slope detecting step of FIG. 1 has some errors. The errors cause the deterioration of the picture since the errors appear as noise in the interpolated screen.